Conventional electrochemical cells, such as alkaline cells, are formed of a cylindrical housing having an open end and an end cap assembly inserted therein. After the cell contents are supplied, the cell is closed by crimping the housing edge over the end cap assembly to provide a seal for the cell. The end cap assembly comprises an exposed end cap plate which functions as a cell terminal and typically a plastic insulating member which seals the open end of the cell housing. A problem associated with design of various electrochemical cells, particularly alkaline cells, is the tendency of the cell to produce gases as it continues to discharge beyond a certain point, normally around the point of complete exhaustion of the cell's useful capacity. Electrochemical cells, particularly alkaline cells, are conventionally provided with rupturable diaphragms or membranes within an end cap assembly. The rupturable diaphragm or membrane may be formed within a plastic insulating member as described, for example, in U.S. Pat. No. 3,617,386. Such diaphragms are designed to rupture when gas pressure within the cell exceeds a predetermined level. The end cap assembly may be provided with vent holes for the gas to escape when the diaphragm or membrane is ruptured. The end cap assembly disclosed in U.S. Pat. No. 3,617,386 uses considerable space above the rupturable diaphragm which reduces the amount of available space within the cell for active material. Also, the end cap assembly disclosed in the reference is not designed to withstand radial compressive forces and will tend to leak when the cell is subjected to extremes in hot and cold climate.
In order to provide a tight seal the prior art discloses end cap assemblies which include a metal support disk inserted between the end cap plate and an insulating member, typically a plastic insulating disk which electrically insulates the metal support disk from the cell housing. The metal support disk may have a highly convoluted surface as shown in U.S. Pat. Nos. 5,532,081 or 5,080,985 which assures that end cap assembly can withstand high radial compressive forces during crimping of the cell's housing edge around the end cap assembly. Such support disk allows the radial forces to be maintained. This results in a tight mechanical seal around the end cap assembly at all times.
Also, the prior art discloses rupturable vent membranes which are integrally formed as part of an insulating disk included within the end cap assembly. Such vent membranes are typically in the form of a rupturable disk which lies in a plane perpendicular to the cell's longitudinal axis, for example, as shown in U.S. Pat. No. 4,537,841 or PCT patent application publication WO 00/46864. The rupturable thin portion within the insulating member may also take the form of a circumferential vent membrane as disclosed in U.S. Pat. No. 5,080,985. As shown in these three references the rupturable membrane lies in a plane perpendicular to the cell's longitudinal axis. Also, as shown in these references, there is required considerable free space above the rupturable disk to allow the disk to rupture cleanly and allow gas to escape therethrough. The disadvantage of such designs is that the significant free space above the rupturable membrane represents a void volume which cannot be used for anode and cathode active material. Thus, the potential cell capacity is reduced by the amount of such free space.
U.S. Pat. No. 6,127,062 discloses an insulating sealing disk and an integrally formed rupturable membrane which is oriented vertically, that is, parallel to the cell's central longitudinal axis. The rupturable membrane is straight walled, that is, is not notched, and is disposed against an opening in an overlying metal support member. When the gas pressure within the cell rises to a predetermined level the membrane penetrates into the opening and ruptures thereby releasing the gas pressure. The size of the overlying opening to achieve good outflow of gas when the membrane ruptures is limited by the vertical orientation of the membrane.
Accordingly, it is desirable to have an end cap assembly which provides a tight seal for the cell even though the cell may be exposed to extremes in both hot and cold climate.
It is desirable that the rupturable venting mechanism occupy minimal amount of space within the cell so that the cell can be filled with additional amounts of anode and cathode material, thereby increasing the cell's capacity.
It is desired that and rupturable venting mechanism be readily manufactured so that venting occurs at a specific predetermined pressure level.